The present invention relates to a primary source with at least two channels, called a monopulse source such as, for example, a Cassegrain or lens type reflector antenna connected to a microwave transmission and reception circuit. It can be applied especially to millimeter wave radars fitted into automobiles. More generally, it can be applied to millimeter wave radars requiring a high level of integration and low-cost manufacture.
A source known as a monopulse source has for example two channels and simultaneously generates two radiation patterns, a sum pattern and a difference pattern. This source must have radioelectrical sources that are compatible with the matching and radiation performance characteristics of a full focal feed antenna. These characteristics relate in particular to the matching frequency band, the formation of the pattern of the difference channel in the plane of the electrical field E and the apertures and the relative level of the radiation patterns of the sum and difference channels.
In certain applications such as automobile vehicles for example, the source must furthermore comply with, technological and economic criteria, both general and specific. These criteria are as follows:
ease of connection and installation as close as possible to the microwave transmission and reception circuit which is made by microstrip technology, so as to minimize the lengths of the lines where the major losses in the millimeter wave band, for example in the range of 80 dB, can soon place limits on the performance characteristics of the system; PA1 the shielding of the microwave transmission and reception circuit against external electromagnetic effects outside the operating band of the system; PA1 the compactness in depth of the primary source which should have, for example, a depth of less than 5 mm; PA1 the imperviousness and possibly hermetically sealed character of the transmission and reception circuits with respect to external environmental effects with the assembly constituted by the transmission and reception circuit and the primary source; PA1 manufacture by conventional manufacturing means and tolerance in operation with respect to dimensional variations obtained with these manufacturing means within the context of very low-cost mass production.
One way of making a primary source that meets certain of the above criteria consists of the use of a pyramidal horn excited by a magic-T circuit folded in the plane of the electrical field E. Depending on the access used, this magic-T circuit is used for the generation, in the horn, of the transverse-electric mode TE01, namely the even mode, or the transverse-magnetic mode TM11, namely the odd mode. These modes respectively form the sum and difference patterns. However, this approach entails a large space requirement in depth and, in order to be made, calls for the manufacture and assembly of several high-precision parts leading to the use of expensive machining methods such as wire electroerosion or electroforming.
In another approach, a printed circuit source is made on the same substrate as the microwave emission circuit. To form radiation patterns with the desired directivity, this source should be formed by an array of patch type radiating elements fed for example by a hybrid ring. This approach has the advantage of not requiring any mechanical parts and of taking up minimum space in depth, but does not meet the requirements of electromagnetic shielding and protection against environmental effects for the components of the microwave transmission and reception circuit. Furthermore, patch type radiating elements have frequency selective operation and are therefore highly sensitive to the characteristics of the substrate, especially for example its dielectric constant or its thickness, and also to the etching tolerance characteristics.